This invention relates to electrical measuring and counting devices, particularly to such devices known in the art as universal counter timers adapted for measuring frequency and time intervals between electrical signals.
Universal counter timers ("UCTs") are typically employed to measure frequency and time intervals between two electrical signals. For time interval measurements, a first electrical signal is provided to a first channel of the UCT and a second electrical signal is provided to a second channel of the UCT. The UCT is typically employed to measure the elapsed time between the time the first signal rises to a predetermined amplitude and the time the second signal rises to the same, predetermined amplitude. If the first electrical signal and second electrical signal are periodic and have the same frequency, the measured time delay may be compared to the period of the signals and a phase delay may be derived from the time delay and period. If the signals are a periodic, it is often desirable simply to determine the time delay between a selected portion of the first signal and a selected portion of the second signal.
It has been a problem in UCTs generally to measure time delays that are small, e.g., on the order of the set-up and hold time of the UCT. Such time delays are manifest in, inter alia, the phase delay between periodic signals that are coincident or nearly coincident in time (hereinafter "coincident"). The UCT typically determines the time delay between two signals by forming a pulse having a width representative thereof, and measuring the pulse width. But as the pulse width narrows, the frequency spectrum of the pulse is increased. Therefore, the bandwidth of the downstream measuring circuitry generally needs to be increased commensurately. Thence, there is generally a hardware cost penalty to accommodating the measurement of very small time delays using this approach. Moreover, there is generally an absolute lower limit on the time delay between two signals which can be measured. As the speed of electronic hardware has increased, the problem has been minimized; however, at the same time it often becomes more desirable to measure smaller delays. Thence, the problem still remains to an often unacceptable degree.
Often, the manufacturer of the UCT refers to this "dead zone" and warns the user that measurements there within are not reliable. However, some manufacturers fail to acknowledge the dead zone and claim instead that measuring phase delay anywhere within the full 360 degree range is permissible when, in fact, it is not. In any case, a practical result of attempting to measure, with prior art UCTs, small time delays, is to obtain erroneous measurements. Moreover, the UCT generally gives no indication that the delay between input signals meets a condition that causes the UCT to produce erroneous measurements. Rather, the user is left to infer this condition from the reported measurement and its perceived unlikeliness in the context of the user's understanding and expectation.
One attempt at solution of the aforementioned problem known in the art is to delay one or the other signal by a predetermined amount and account for the predetermined delay in subsequent computation. Thence, if the time delay between the two signals is less than the minimum pulse width that can be measured in the UCT, an appropriate predetermined delay may be employed to widen the pulse so that the dead zone is avoided and, theoretically, a time delay may be reported that is equal to zero.
However, a predetermined delay that is effective to move one pair of signals out of the dead zone may move another pair of signals into the dead zone. For example, a predetermined delay that moves a first pair of signals having a first phase relationship out of the dead zone will cause another pair of signals having a phase relationship that is reversed from the first phase relationship to move into the dead zone. In the same way, a predetermined delay imposed between a pair of signals will move the signals out of the dead zone if the signals have the same phase; however, the signals will be moved into the dead zone if the signals differ in phase by an amount corresponding to the predetermined delay. Further, if the predetermined delay happens to be equal to the period of periodic signals, the delay will have no effect on the signals. It can be appreciated that a predetermined delay imposed between a pair of signals cannot be relied upon with certainty to remove coincidence where the signals are permitted to have arbitrary phase relationships and periods.
Where it is desired to measure the phase delay of two periodic signals having the same frequency, the UCT must determine both the time delay between the two signals and the period of the signals in order to express a result in, typically, degrees of phase. Accordingly, the time delay and the period are inherently temporally related in a measurement of phase delay. This is particularly important when it is desired to know the phase delay at a particular time, such as in gating while making simultaneous measurements. Notwithstanding this temporal relationship, time delay and period are generally determined sequentially in UCTs or, if measured coincidently, are determined with different hardware. In the former case, inaccuracy may be introduced by employing two different periods of a signal for determining the two parameters. In the latter case, the expense of additional hardware must generally be incurred.
Accordingly, there is a need for a method and apparatus for measuring time intervals between electrical signals that provides for accurate measurement of the time intervals over an entire range of possible time or phase delays, including small time or phase delays, and that provides for accurate measurement of the phase delay between two periodic signals.